Methods for recovering nitrogen from process gas streams

ABSTRACT

Methods and systems for recovering nitrogen and alkenes (e.g. ethylene, propylene) from process gas streams, including multi-step condensing of the process gas stream, are provided herein.

FIELD OF THE INVENTION

This invention relates to methods for recovering nitrogen and alkenes from various process gas streams, particularly petrochemical vent gas streams.

BACKGROUND OF THE INVENTION

Valuable hydrocarbons, such as lower alkenes, may be present in a multitude of process gas streams at varying concentrations and typically amongst other valuable components, such as nitrogen, as well as impurities. For example, various petrochemical vent gas streams, such as those produced during polymerization processes, may contain ethylene, propylene and/or inert purge gases, such as nitrogen, as well as methane, ethane, propane, hydrogen, other higher hydrocarbons and other impurities. Thus, it would be desirable to be able to recover such valuable components, such as alkenes and nitrogen, for reuse in the process. However, typically, such streams containing alkene process gas (e.g., ethylene, propylene) are removed from the system via flaring or with a thermal oxidizer, which prevents recovery of the alkenes and reuse of nitrogen. Furthermore, environmental regulations limit the amount of NOx compounds as well as the hydrocarbon flare products that can be emitted. Prior to flaring or thermal oxidizing, recovery processes may be employed for recovering as much of the alkene as possible so it is not lost when flared or oxidized. Examples of existing recovery processes include compression and condensation systems as well as use of pressure swing adsorption systems and/or membranes. However, existing recovery processes may still be limited in the amount of nitrogen and alkene recovery.

Thus, a need remains for improved and more effective methods for recovery of nitrogen and alkenes from process gas streams.

SUMMARY OF THE INVENTION

It has been found that nitrogen and alkene gas may be sufficiently recovered by performing a combination of process steps including cooling a process gas stream to remove a portion of the impurities via a condensate stream followed by further cooling of the process gas stream for nitrogen and alkene recovery.

Thus, in one aspect, this disclosure relates to a method for recovering nitrogen from a process gas stream comprising: cooling the process gas stream comprising nitrogen and C₂-C₄ alkene in a first condenser vessel with a first cooling medium under suitable conditions to produce a first condensate comprising impurities, a first cooled process gas stream comprising nitrogen and C₂-C₄ alkene, and a first gaseous nitrogen stream; cooling the cooled process gas stream in a second condenser vessel with a second cooling medium under suitable conditions to produce a second condensate comprising C₂-C₄ alkene, a nitrogen-containing vent gas stream, and second gaseous nitrogen stream; and combining at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream.

In still another aspect, this disclosure relates to a system recovering nitrogen gas from a process gas stream comprising: a process gas stream comprising nitrogen and C₂-C₄ alkene; a first cooling medium stream; a first condensate stream comprising impurities; a cooled process gas stream comprising nitrogen and C₂-C₄ alkene; a first gaseous nitrogen stream; a second cooling medium stream; a second condensate stream comprising C₂-C₄ alkene; a nitrogen-containing vent gas stream; a second gaseous nitrogen stream; a combined nitrogen stream; a first condenser vessel operated under suitable conditions to produce the first condensate stream comprising impurities, the cooled process gas stream comprising nitrogen and C₂-C₄ alkene, and the first gaseous nitrogen stream, wherein the first condenser vessel comprises: a first heat exchanger; a first inlet for providing the process gas stream; a second inlet for providing the first cooling medium stream; a first outlet for removal of the first gaseous nitrogen stream; a second outlet for removal of the first condensate stream; and a third outlet for removal of the cooled process gas stream; a second condenser vessel operated under suitable conditions to produce the second condensate stream comprising C₂-C₄ alkene, the nitrogen-containing vent gas stream, and second gaseous nitrogen stream, wherein the second condenser vessel comprises: a second heat exchanger; a fourth inlet for providing the cooled process gas stream: a fifth inlet for providing the second cooling medium stream; a fourth outlet for removal of the nitrogen-containing vent gas stream; a fifth outlet for removal of the second gaseous nitrogen stream; and a sixth outlet for removal of the second condensate stream; and a nitrogen vessel comprising at least two of: a seventh inlet for providing the first gaseous nitrogen stream; an eighth inlet for providing the nitrogen-containing vent gas stream; a ninth inlet for providing the second gaseous nitrogen stream; and seventh outlet for removal of the combined nitrogen stream.

Other embodiments, including particular aspects of the embodiments summarized above, will be evident from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 illustrates a schematic of a system for recovering nitrogen and alkenes from process gas streams according to certain aspects of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

In various aspects of the invention, methods and systems for recovering nitrogen and alkenes from process gas streams are provided.

I. Definitions

The term “and/or” as used in a phrase such as “A and/or B” herein is intended to include “A and B”, “A or B”, “A”, and “B”.

As used herein, and unless otherwise specified, the term “C_(n)” means hydrocarbon(s) having n carbon atom(s) per molecule, wherein n is a positive integer. As used herein, and unless otherwise specified, the term “hydrocarbon” means a class of compounds containing hydrogen bound to carbon, and encompasses (i) saturated hydrocarbon compounds, (ii) unsaturated hydrocarbon compounds, and (iii) mixtures of hydrocarbon compounds (saturated and/or unsaturated), including mixtures of hydrocarbon compounds having different values of n.

As used herein, the term “alkene,” alternatively referred to as “olefin,” refers to a branched or unbranched unsaturated hydrocarbon having one or more carbon-carbon double bonds. A simple alkene comprises the general formula C_(n)H_(2n), where n is 2 or greater. Examples of alkenes include, but are not limited to ethene, propene, butene, pentene, hexene and heptene. “Alkene” is intended to embrace all structural isomeric forms of an alkene. For example, butene encompasses but-1-ene, (Z)-but-2-ene, etc.

As used herein, the term “oxygenate,” refers to refers to oxygen-containing compounds having 1 to about 20 carbon atoms, 1 to about 10 carbon atoms, or 1 to about 4 carbon atoms. Exemplary oxygenates include alcohols, ethers, carbonyl compounds, e.g., aldehydes, ketones and carboxylic acids, and mixtures thereof. Particular non-limiting examples of oxygenates include methanol, ethanol, dimethyl ether, diethyl ether, methylethyl ether, di-isopropyl ether, dimethyl carbonate, dimethyl ketone, formaldehyde, acetaldehyde, acetic acid, and the like, and combinations thereof.

II. Methods for Recovering Nitrogen and Alkenes from Process Gas Streams

Methods for recovering nitrogen and alkene from process gas streams are provided herein. In particular, the process gas streams may comprise one or more petrochemical vent gas streams, such as but not limited to vent gas streams produced during polymerization productions, e.g., monomer supply and purification vent gas streams, polymerization reactor vent gas streams, and resin cleaning vent gas streams. As discussed above, a process gas stream may comprise various impurities in addition to alkene gas and nitrogen gas. Thus, it is necessary to efficiently and effectively remove these impurities from the process gas stream in order to sufficiently recover nitrogen and alkenes.

Therefore, in order to remove at least a portion of impurities present in a process gas stream, a process gas stream may be cooled in a first condenser vessel with a first cooling medium under suitable conditions to produce a first condensate stream comprising at least a portion of the impurities, a first cooled process gas stream comprising nitrogen and alkene (e.g., C₂-C₄ alkenes), and a first gaseous nitrogen stream. The first condensate stream may be collected in a first condensate tank.

The process gas stream comprises alkene gas, e.g., C₂-C₁₀ alkenes, C₂-C₈ alkenes or C₂-C₄ alkenes. In particular the process gas stream comprises ethylene and/or propylene. Additionally, the process gas stream may comprise alkene gas (e.g., C₂-C₄ alkenes), in an amount, based on the total weight of the process gas stream, of less than or equal to about 99 wt %, less than or equal to about 95 wt %, less than or equal to about 90 wt %, less than or equal to about 85 wt %, less than or equal to about 80 wt %, less than or equal to about 75 wt %, less than or equal to about 70 wt %, less than or equal to about 65 wt %, less than or equal to about 60 wt %, less than or equal to about 55 wt %, less than or equal to about 50 wt %, less than or equal to about 45 wt %, less than or equal to about 40 wt %, less than or equal to about 35 wt %, less than or equal to about 30 wt %, less than or equal to about 25 wt %, less than or equal to about 20 wt %, less than or equal to about 15 wt %, less than or equal to about 10 wt %, or about 1.0 wt %. In particular, the process gas stream may comprise alkene gas (e.g., C₂-C₄ alkenes), singularly or in combination, in an amount, based on the total weight of the process gas stream, of less than or equal to 80 wt %. It is understood herein that the amount of alkene gas provided herein corresponds to both a single alkene amount as well as combined amounts of alkenes, if one or more are present. For example, an alkene gas (e.g., C₂-C₄ alkenes) present in an amount of less than or equal to 80 wt % encompasses a process stream comprising less than or equal to 80 wt % ethylene as well as a process stream comprising less than or equal to 80 wt % ethylene and propylene in combination. Additionally or alternatively, the process gas stream may comprise alkene gas (e.g., C₂-C₄ alkenes) in an amount, based on the total weight of the process gas stream, of about 1.0 wt % to about 99 wt %, about 1.0 wt % to about 90 wt %, about 1.0 wt % to about 85 wt %, about 1.0 wt % to about 80 wt %, about 1.0 wt % to less than about 80 wt %, about 1.0 wt % to about 60 wt % or about 10 wt % to about 50 wt %.

A remaining portion, e.g., the balance, of the process gas stream may further comprise nitrogen and/or impurities, such as, but not limited C₁-C₂₀ hydrocarbons (e.g., methane, ethane, propane, butane, butene, pentane, pentene, hexane, hexene, etc.), hydrogen, water and/or oxygenates (e.g., methyl acetate, ethyl acetate, vinyl acetate, methanol, dimethyl ether, acetaldehyde, etc.). In particular, the impurities may be selected from the group consisting of nitrogen, methane, ethane, propane, butane, pentane, pentene, hexane, hexene, hydrogen, methyl acetate, ethyl acetate, vinyl acetate, methanol, dimethyl ether, acetaldehyde, water, and a combination thereof. For example, the nitrogen and/or impurities may be present in the process gas stream in amount of at least about 1.0 wt %, at least about 5.0 wt %, at least about 10 wt %, at least about 15 wt %, at least bout 20 wt %, at least about 25 wt %, at least about 30 wt %, at least about 35 wt %, at least about 40 wt %, at least about 45 wt %, at least about 50 wt %, at least about 55 wt %, at least about 60 wt %, at least about 65 wt %, at least about 70 wt %, at least about 75 wt %, at least about 80 wt %, at least about 85 wt %, at least about 90 wt %, at least about 95 wt or about 99 wt %. Additionally or alternatively, the nitrogen and/or impurities may be present in the process gas stream in amount of about 1.0 wt % to about 99 wt %, about 15 wt % to about 99 wt %, about 20 wt % to about 99 wt %, about 20 wt % to about 90 wt %, about 30 wt % to about 80 wt %, about 40 wt % to about 75 wt %, about 1.0 wt % to about 40 wt %, about 1.0 wt % to about 20, about 1.0 wt % to about 10 wt %, or about 1.0 wt % to about 5.0 wt. %. It is understood herein that the amount of impurities provided herein corresponds to both a single impurity amount as well as combined amounts of impurities, if one or more are present. For example, impurities present in an amount of at least about 10 wt % encompasses a process stream comprising at least about 10 wt % methanol as well as a process stream comprising at least about 10 wt % methanol and methane in combination.

In a particular embodiment, the process gas stream may comprise at least about 1.0 wt % nitrogen and less than about 80 wt % alkene gas (e.g., C₂-C₄ alkenes).

In various aspects, the process gas stream may enter the first condenser vessel at any suitable temperature and/or pressure, for example, as determined by previous process steps and conditions for producing the process gas stream. For example, the process gas stream may enter the first condenser vessel at a temperature of about 50° C. or lower, about 40° C. or lower, about 30° C. or lower, about 20° C. or lower, about 10° C. or lower, about 0.0° C. or lower, about −10° C. or lower, about −20° C. or lower, about −30° C. or lower, about −40° C. or lower, or about −50° C. Additionally or alternatively, the process gas stream may enter the first condenser vessel at a temperature of about −50° C. to about 50° C., about −50° C. to about 40° C., about −50° C. to about 20° C., about −50° C. to about 0.0° C., about −50° C. to about −10° C., about −50° C. to about −20° C., about −50° C. to about −30° C., about −50° C. to about −40° C., about −40° C. to about −0.0° C., about −40° C. to about −10° C., about −40° C. to about −20° C., about −40° C. to about −30° C., about −30° C. to about 50° C., about −10° C. to about 50° C., or about 0.0° C. to about 40° C. Additionally, the process gas stream may enter the first condenser vessel at a pressure, optionally in combination with the above-described temperatures, of at least about 80 kPa, at least about 90 kPa, at least about 100 kPa, at least about 110 kPa, at least about 120 kPa, at least about 150 kPa, at least about 180 kPa, at least about 200 kPa, at least about 220 kPa, at least about 250 kPa, at least about 280 kPa, or about 300 kPa. For example, the process gas stream may enter the first condenser vessel at a pressure, optionally in combination with the above-described temperatures, of about 80 kPa to about 300 kPa, about 90 kPa to about 250 kPa, about 90 kPa to about 200 kPa, about 90 kPa to about 150 kPa or about 90 kPa to about 110 kPa. In particular, the process gas stream may enter the first condenser vessel at a temperature of about 40° C. or lower and a pressure of at least about 90 kPa.

In order to produce the first condensate stream, a first cooling medium may be circulated through the first condenser vessel at a temperature suitable for condensing at least a portion of the impurities present in the process gas stream to produce the first condensate stream comprising at least a portion of the impurities, the cooled process gas stream comprising nitrogen and alkenes (e.g., C₂-C₄ alkenes), and a first gaseous nitrogen stream, all which may exit the first condenser vessel. Thus, the first cooled process gas stream may have impurities present in an amount less than an amount of impurities present in the process gas stream entering the first condenser vessel. Further, the cooled process gas stream exiting the first condenser vessel may have a temperature of less than or equal to about 0.0° C., less than or equal to about −10° C., less than or equal to about −20° C., less than or equal to about −30° C., less than or equal to about −40° C., less than or equal to about −50° C., less than or equal to about −60° C., less than or equal to about −70° C., less than or equal to about −80° C., less than or equal to about −90° C., less than or equal to about −100° C., less than or equal to about −110° C., or about −120° C. Additionally or alternatively, the first cooled process gas stream may have a temperature of about −120° C. to about 0.0° C., about −120° C. to about −10° C., about −120° C. to about −20° C., about −120° C. to about −30° C., about −120° C. to about −40° C. or about −110° C. to about −50° C.

In order to further recover nitrogen and alkenes (e.g., C₂-C₄ alkene) from the process gas stream, the method may further comprise cooling the cooled process gas stream comprising nitrogen and alkenes (e.g., C₂-C₄ alkene) in a second condenser vessel under suitable conditions to produce a second condensate comprising alkene (e.g., C₂-C₄ alkenes), a nitrogen-containing vent gas stream and a second gaseous nitrogen stream. A second cooling medium may be circulated through the second condenser vessel at a temperature suitable for condensing at least a portion of the alkene gas (e.g., C₂-C₄ alkenes) present in the process gas stream to produce the second condensate comprising alkenes (e.g., C₂-C₄ alkenes). The second condensate comprising alkenes (e.g., C₂-C₄ alkenes) may be collected in a second condensate tank. Additionally or alternatively, the second condensate exiting the second condenser vessel may have a temperature of about −170° C. to about −104° C., about −170° C. to about −120° C., or about −170° C. to about −140° C., or about −150° C. to about −104° C.

In various aspects, the second condensate may comprise at least about 40 wt %, at least about 50 wt %, at least about 60 wt %, at least about 70 wt %, at least about 80 wt %, at least about 90 wt %, at least about 95 wt %, or about 99 wt % of the alkenes (e.g., C₂-C₄ alkenes), which were present in the process gas stream. For example, the second compensate may comprise about 40 wt % to about 99 wt %, about 50 wt % to about 99 wt %, about 70 wt % to about 99 wt %, about 80 wt % to about 99 wt %, or about 90 wt % to about 99 wt % of the alkenes (e.g., C₂-C₄ alkenes), which were present in the process gas stream, In particular, at least about 80 wt % of the alkenes (e.g., C₂-C₄ alkenes) present in the process gas stream may be present in the second condensate.

Examples of suitable first and second cooling mediums include, but are not limited to liquid nitrogen and/or gaseous nitrogen. As the first and second cooling medium (e.g., liquid and/or gaseous nitrogen) circulate and provide cooling through the first and second condenser vessels, respectively, they can become heated and exit the first and second condenser vessels as a first gaseous nitrogen stream and a second gaseous nitrogen stream, respectively. The first and/or second cooling medium (e.g., liquid and/or gaseous nitrogen) may be provided and/or circulated in the first and/or second condenser vessel at a temperature of at least about −200° C., at least about −196° C., at least about −190° C., at least about −180° C., at least about −170° C., at least about −160° C., or at least about −150° C. For example, the first and/or second cooling medium (e.g., liquid and/or gaseous nitrogen) may be provided and/or circulated in the first and/or second condenser vessel at a temperature of about −200° C. to about −150° C., about −196° C. to about −150° C., about −190° C. to about −150° C., or about −180° C. to about −160° C. Additionally, the first and/or second cooling medium (e.g., liquid nitrogen and/or gaseous nitrogen) may be provided and/or circulated in the first and/or second condenser vessel, optionally in combination with the above-described temperatures, at a pressure of less than or equal to about 1600 kPa, less than or equal to about 1500 kPa, less than or equal to about 1300 kPa, less than or equal to about 1200 kPa, less than or equal to about 1000 kPa, less than or equal to about 800 kPa, less than or equal to about 700 kPa, less than or equal to about 500 kPa, less than or equal to about 300 kPa, or about 100 kPa. For example, the first and/or second cooling medium (e.g., liquid and/or gaseous nitrogen) may be provided and/or circulated in the in the first and/or second condenser vessel, optionally in combination with the above-described temperatures, at a pressure of about 100 kPa to about 1600 kPa, about 300 kPa to about 1500 kPa, about 500 kPa to about 1500 kPa, about 700 kPa to about 1300 kPa or about 800 kPa to about 1200 kPa. In particular, the first and/or second cooling medium (e.g., liquid and/or gaseous nitrogen) may be provided and/or circulated in the first and/or second condenser vessel at a temperature of at least about −196° C. and/or at a pressure of less than or equal to about 1500 kPa (e.g., about −170° C. and about 1000 kPa). The first and/or second condenser vessel may each independently comprise heat exchangers, such as a coil heat exchanger, a shell and tube heat exchanger and a plate heat exchanger.

As discussed above, a nitrogen-containing vent gas may also be produced in the second condenser vessel. In addition to nitrogen, the nitrogen-containing vent gas stream may comprise other non-condensable components of the process gas stream. For example, the nitrogen-containing vent gas stream may primarily comprise (e.g., ≥about 90 wt %, ≥about 95 wt %, ≥about 98 wt %, ≥about 99 wt %, or about 99.5 wt %) nitrogen, singularly or in combination with, hydrogen, water, and the impurities (e.g., methane, ethane) as described herein present in the process gas stream. In particular, the nitrogen-containing vent gas stream comprises nitrogen in an amount of at least about 70 wt %, at least about 80 wt %, at least about 90 wt %, at least about 95 wt %, at least about 98 wt %, at least about 99 wt % or about 99.4 wt %, based on the total weight of the nitrogen-containing vent gas stream. Additionally or alternatively, the nitrogen-containing vent gas stream may comprise trace amounts (e.g., ≤about 5.0 wt %, ≤about 2.0 wt %) of alkenes (e.g., C₂-C₄ alkenes). Further, during the process, the nitrogen-containing vent gas stream may be continuously removed from the second condenser vessel or when the pressure in the second condenser vessel reaches a predetermined value. For example, the nitrogen-containing vent gas may be removed from the second condenser vessel when the pressure in the second condenser vessel reaches greater than about 100 kPa, at least about 200 kPa, at least about 300 kPa, at least about 400 kPa or about 500 kPa. Additionally or alternatively, the nitrogen-containing vent gas may be removed from the second condenser vessel when the pressure in the condenser vessel is from above about 100 kPa to about 500 kPa, from above about 200 kPa to about 400 kPa, or about 300 kPa to about 500 kPa. Further, the nitrogen-containing nitrogen-containing vent gas may exit the second condenser vessel at a temperature of about −170° C. to about −104° C., or about −160° C. to about −104° C. and/or at pressure of at least about 100 kPa, at least about 200 kPa, at least about 300 kPa, at least about 400 kPa, or at least about 500 kPa.

Additionally, at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream may be combined, for example in a tank or a gaseous nitrogen distribution system, to form a combined nitrogen stream. This combined nitrogen stream may then be recycled and reused as needed by the process. In one embodiment, the first gaseous nitrogen stream and the nitrogen-containing vent gas stream may be combined. In another embodiment, the first gaseous nitrogen stream and the second gaseous nitrogen stream may be combined. In another embodiment, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream may be combined. In another embodiment, the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream may be combined. Advantageously, the combination of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream, i.e., the combined nitrogen stream, may comprise nitrogen, based on total weight of the combination, in an amount greater than the nitrogen-containing vent gas stream. Further the combined nitrogen stream advantageously may comprise trace amounts (e.g., ≤about 2.0 wt %, ≤about 1.0 wt %, ≤about 0.5 wt %, 5 about 0.25 wt %, ≤about 0.10 wt %) of alkenes (e.g., C₂-C₄ alkenes), which can allow for reuse of the combined nitrogen stream and can avoid flaring. Additionally or alternatively, the combined nitrogen stream may comprise nitrogen in an amount, based on the total weight of the combined nitrogen stream, of ≥about 80 wt %, ≥about 90 wt %, ≥about 95 wt %, about 98 wt %, about 99 wt %, or about 99.9 wt %. For example, the combined nitrogen stream may comprise nitrogen in an amount, based on the total weight of the combined nitrogen stream, of about 80 wt % to about 99.9 wt %, about 90 wt % to about 99.5 wt % or about 95 wt % to about 99.9 wt %. In particular, the combined nitrogen stream may comprise nitrogen in an amount, based on the total weight of the combined nitrogen stream, of ≥about 90 wt %.

A. Optional Additional Steps

Optionally, the condensation cycle may further comprise cooling the process gas stream in a pre-cooling apparatus prior to the process gas entering the first condenser vessel. Suitable pre-cooling apparatuses include, but are not limited to a mechanical chiller, such as a water chiller. In the pre-cooling apparatus, the process gas stream may be cooled to a temperature of about −40° C. or higher, −30° C. or higher, −10° C. or higher, 0.0° C. or higher, 10° C. or higher, 20° C. or higher, 30° C. or higher, 40° C. or higher, or about 50° C. In particular, the process gas stream in the pre-cooling apparatus may be cooled to a temperature of about −40° C. or higher. Additionally or alternatively, in the pre-cooling apparatus, the process gas stream may be cooled to a temperature of about −40° C. to about 50° C., about −20° C. to about 50° C., or about 0.0° C. to about 50° C.

Optionally, the method may further comprise pressurizing the second condensate stream. For example, the method may comprise halting flow of the process gas stream and the first cooling medium to the first condenser vessel, flow of the first condensate to the first condensate tank, flow of the first cooled process gas stream and the second cooling medium to the second condenser vessel and/or flow of the second condensate to the second condensate tank. Optionally, the process gas stream may be directed to another condenser vessel in series where it may undergo cooling as described herein. Further, heat may be applied to the second condensate tank to produce pressurized liquid alkene (e.g., C₂-C₄ alkene) and an alkene (e.g., C₂-C₄ alkene) vent gas. The heat provided to the at least one condensate tank may vaporize at least a portion of the alkenes (e.g., C₂-C₄ alkene) in the first condensate to produce pressurized liquid alkene (e.g., C₂-C₄ alkene) and an alkene (e.g., C₂-C₄ alkene) vent gas. The alkene (e.g., C₂-C₄ alkene) vent gas may also comprise other components, such as ethane and/or methane. Any suitable means for providing heat to the second condensate tank may be used, for example, heat may be provided via a heater (e.g., electric heater), via heated gaseous nitrogen, via heated air or via an ambient vaporizer. Further, the heat may be provided at suitable temperature for a suitable amount of time to produce pressurized liquid alkene (e.g., C₂-C₄ alkene) at a desirable pressure as determined by the needs of the process, for example, at a pressure of about 100 kPa to about 1500 kPa, about 200 kPa to about 800 kPa or about 300 kPa to about 500 kPa. For example, the pressurized liquid alkene (e.g., C₂-C₄ alkene) may be maintained at a pressure of at least about 100 kPa. Additionally or alternatively, the pressurized liquid alkene (e.g., C₂-C₄ alkene) may have a temperature of less than about −104° C., e.g., about −170° C. to about −104° C., about −170° C. to about −120° C., about −170° C. to about −140° C., or about −150° C. to about −104° C. The pressurized liquid alkene (e.g., C₂-C₄ alkene) may then be recycled and reused as needed in the process.

The methods described herein may further comprise a defrost cycle to melt any frozen alkenes and/or impurities in the condenser vessels (e.g., first condenser vessel, second condenser vessel). The defrost cycle may be performed as needed by the process. For example, the defrost cycle may be commenced when the pressure drop within the condenser vessels (e.g., first condenser vessel, second condenser vessel) increases to an undesirable level and/or when the level of condensate in the condensate tanks (e.g., first condensate tank, second condensate tank) is too high. The defrost cycle may comprise halting the flow of the process gas stream and the first cooling medium to the first condenser vessel, flow of the first condensate to the first condensate tank, flow of the cooled process gas stream and the second cooling medium to the second condenser vessel and/or flow of the second condensate to the second condensate tank. Then the condenser vessels (e.g., first condenser vessel, second condenser vessel) may be heated to produce at least a third condensate stream comprising alkenes (e.g., C₂-C₄ alkene). The heating of the condenser vessels (e.g., first condenser vessel, second condenser vessel) may be provided by any suitable means for defrosting the condenser vessel. For example, heating may be provided by gaseous nitrogen. The gaseous nitrogen may be heated to a suitable temperature (e.g., greater than about −104° C. up to about 25° C.) prior to introduction into the condenser vessels (e.g., first condenser vessel, second condenser vessel). The defrost cycle may further comprise draining the third condensate stream from the second condenser vessel to collect in the second condensate tank or in a third condensate tank.

III. Systems for Recovering Nitrogen and Alkenes from Process Gas Streams

Systems for recovering alkene (e.g., C₂-C₄ alkene) from a process gas stream as described herein are also provided. Referring to FIG. 1, the system 1 may comprise a process gas stream 2, which is provided to a first condenser vessel 3 via a first inlet (not shown), for example, wherein a first condensate stream 4, a cooled process gas stream 5 are produced. In particular, the process gas stream 2 may comprise nitrogen as described herein and alkenes (e.g., C₂-C₄ alkene) as described herein, e.g., the process gas stream 2 may comprise at least about 1.0 wt % nitrogen and less than about 80 wt % alkene gas (e.g., C₂-C₄ alkenes). In certain aspects, the alkenes are ethylene and/or propylene. In various aspects, one or more petrochemical vent gas streams as described herein optionally may be present to form the process gas stream 2. For example, as shown in FIG. 1, petrochemical vent gas streams 30, 31, and 32 may be present in system 1 along with a means 33 for combining the one or more petrochemical vent gas streams 30, 31, and 32 to form the process gas stream 2. Suitable means 33 for combining the petrochemical vent gas streams may include, but is not limited to a vessel, a tank, conduits, piping, gas distribution headers or combinations thereof. In an alternative embodiment, instead of combining the petrochemical vent gas streams 30, 31, and 32 into the process gas stream 2, one or more of the petrochemical vent gas streams 30, 31, and 32 separately may be introduced into the first condenser vessel 3.

Additionally, in order to produce the first condensate stream 4 comprising at least a portion of the impurities as described herein and the cooled process gas stream 5 as described herein (e.g., having a temperature of less than or equal to 0.0° C.), the system 1 may further comprise a first cooling medium stream 6 provided via a second inlet (not shown), as controlled by a valve 7, which may be circulated through the first condenser vessel 3, at a temperature suitable for condensing at least a portion of the impurities present in the process gas stream 2 to produce the first condensate stream 4 comprising impurities as well as the cooled process gas stream 5. The first condenser vessel 3 may comprise, consist essentially of, or consist of a first heat exchanger (e.g., shell and tube heat exchanger, plate heat exchanger, coil heat exchanger). The first cooling medium stream 6 may comprise a suitable cooling medium as described herein, for example, liquid nitrogen and/or gaseous nitrogen. As the first cooling medium stream 6 (e.g., gaseous and/or liquid nitrogen) circulates through the first condenser vessel 3, it may be heated and exit the system 1 as a first gaseous nitrogen stream 8 via a first outlet (not shown). Additionally or alternatively, a valve (not shown) may be present on the first gaseous nitrogen stream 8 for controlling the cooling medium circulating through the first condenser vessel 3.

A first condensate tank 9 may be present in the system 1 for collection of the first condensate stream 4, which may be removed via a second outlet (not shown) in the first condenser vessel 3, for example, as the first condensate stream 4 drains from the first condenser vessel 3 during a condensation cycle (e.g., first condensation cycle, second condensation cycle) as described herein. The first condensate tank 9 may comprise a third inlet (not shown) for providing the first condensate stream 4.

The system 1 may further comprise a second condenser vessel 10 for nitrogen and alkene (e.g., C₂-C₄ alkene) recovery. In particular, the cooled process gas stream 5 may exit the first condenser vessel 3 via a third outlet (not shown) and enter the second condenser vessel 10 via a fourth inlet (not shown). The second condenser vessel 10 may be operated under suitable conditions to produce a second condensate stream 11 comprising alkenes (e.g., C₂-C₄ alkene) and a nitrogen-containing vent gas stream 12 as described herein. The nitrogen-containing vent gas stream 12 may exit the second condenser vessel 10 via a fourth outlet (not shown) and may primarily comprise (e.g., ≥about 90 wt %, ≥about 95 wt %, ≥about 98 wt %, %, ≥about 99 wt %, or about 99.5 wt %) the non-condensable components of the process gas stream 2, e.g., hydrogen, nitrogen and/or methane, particularly nitrogen.

In order to produce the second condensate stream 11 as described herein and the nitrogen-containing vent gas stream 12 as described herein, the system 1 may further comprise a second cooling medium stream 13 provided via a fifth inlet (not shown), as controlled by valve 14, which may be circulated through the second condenser vessel 10, at a temperature suitable for condensing at least a portion of the alkenes (e.g., C₂-C₄ alkene) present in the cooled process gas stream 5 to produce the second condensate stream 11 comprising alkenes (e.g., C₂-C₄ alkene). The second condenser vessel 10 may comprise, consist essentially of, or consist of a second heat exchanger (e.g., shell and tube heat exchanger, plate heat exchanger, coil heat exchanger). The second cooling medium stream 13 may comprise a suitable cooling medium as described herein, for example, liquid nitrogen and/or gaseous nitrogen. As the second cooling medium stream 13 (e.g., gaseous and/or liquid nitrogen) circulates through the second condenser vessel 10, it may be heated and exit the system 1 as a second gaseous nitrogen stream 15 via a fifth outlet (not shown). Additionally or alternatively, a valve (not shown) may be present on the second gaseous nitrogen stream 15 for controlling the cooling medium circulating through the second condenser vessel 10.

A second condensate tank 16 may be present in the system 1 for collection of the second condensate stream 11, which may be removed via a sixth outlet (not shown) in the second condenser vessel 10, for example, as the second condensate stream 11 drains from the second condenser vessel 10. The second condensate tank 16 may comprise a sixth inlet (not shown) for providing the second condensate stream 11.

The system may further comprise a nitrogen vessel 17 (e.g., a gaseous nitrogen distribution system) where a combination of at least two of the first gaseous nitrogen stream 8, the nitrogen-containing vent gas stream 12 and the second gaseous nitrogen stream 15 may be present as combined nitrogen and may exit the nitrogen vessel 17 via a seventh outlet (not shown) as a combined nitrogen stream 18, which may be recycled for use throughout the system, for example, for use throughout a petrochemical plant. The nitrogen vessel 17 may comprise at least two of a seventh inlet (not shown) for providing the first gaseous nitrogen stream 8, an eighth inlet (not shown) for providing the nitrogen-containing vent gas stream 12, and a ninth inlet (not shown) for providing the second gaseous nitrogen stream 15. As shown in FIG. 1, all three of the first gaseous nitrogen stream 8, the nitrogen-containing vent gas stream 12 and the second gaseous nitrogen stream 15 are directed to the nitrogen vessel 17. However, it is also contemplated herein that only two of the first gaseous nitrogen stream 8, the nitrogen-containing vent gas stream 12 and the second gaseous nitrogen stream 15 may be directed to the nitrogen vessel 17. As discussed above, the nitrogen vessel 17 may comprise nitrogen, based on total weight of the combination, in an amount greater than the nitrogen-containing vent gas stream 12. Further the combined nitrogen stream 18 present in the nitrogen vessel 17 advantageously may comprise only trace amounts (e.g., ≤about 2.0 wt %, ≤about 1.0 wt %, ≤about 0.5 wt %, ≤about 0.25 wt %, ≤about 0.10 wt %) of alkenes (e.g., C₂-C₄ alkenes), which can allow for reuse of the combined nitrogen stream and can avoid flaring. Additionally or alternatively, the nitrogen vessel 17 may comprise nitrogen in an amount, based on the total weight of the combined nitrogen stream 18, of ≥about 80 wt %, ≥about 90 wt %, ≥about 95 wt %, ≥about 98 wt %, ≥about 99 wt %, or about 99.9 wt %. For example, the nitrogen vessel 17 may comprise nitrogen in an amount, based on the total weight of the combined nitrogen stream 18, of about 80 wt % to about 99.9 wt %, about 90 wt % to about 99.5 wt % or about 95 wt % to about 99.9 wt %. In various aspects, the nitrogen-containing vent gas stream 12 may comprise at least about 90 wt % nitrogen, based on the total weight of the nitrogen-containing vent gas stream 12, and/or the nitrogen vessel 17 may comprise ≥90 wt %, ≥95 wt %, ≥98 wt %, ≥about 99 wt %, or about 99.9 wt % nitrogen, based on the total weight of at least two of the first gaseous nitrogen stream 8, the nitrogen-containing vent gas stream 12, and the second gaseous nitrogen stream 15.

IV. Further Embodiments

The invention can additionally or alternatively include one or more of the following embodiments.

Embodiment 1

A method for recovering nitrogen from a process gas stream (e.g., comprising one or more petrochemical vent gas streams) comprising: cooling the process gas stream comprising nitrogen and C₂-C₄ alkene (e.g., ethylene and/or propylene) in a first condenser vessel with a first cooling medium (e.g., liquid and/or gaseous nitrogen) under suitable conditions to produce a first condensate comprising impurities (e.g., hydrogen, water, C₁-C₂₀ hydrocarbons, and/or oxygenates), a first cooled process gas stream comprising nitrogen and C₂-C₄ alkene (e.g., ethylene and/or propylene), and a first gaseous nitrogen stream; cooling the cooled process gas stream in a second condenser vessel with a second cooling medium (e.g., liquid and/or gaseous nitrogen) under suitable conditions to produce a second condensate comprising C₂-C₄ alkene (e.g., ethylene and/or propylene), a nitrogen-containing vent gas stream, and second gaseous nitrogen stream; and combining at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream.

Embodiment 2

The method of embodiment 1, wherein the process gas stream comprises at least about 1.0 wt %, nitrogen and less than about 80 wt % C₂-C₄ alkene.

Embodiment 3

The method of embodiment 1 or 2, wherein the nitrogen-containing vent gas stream comprises at least about 90 wt % nitrogen based on the total weight of the nitrogen-containing vent gas stream.

Embodiment 4

The method of any one of the previous embodiments, wherein the first condenser vessel and/or the second condenser vessel each independently comprises a coil heat exchanger, a shell and tube heat exchanger or a plate heat exchanger.

Embodiment 5

The method of any one of the previous embodiments, wherein the first cooling medium and/or the second cooling medium is provided at a temperature at least about −196° C. and a pressure of less than or equal to about 1500 kPa.

Embodiment 6

The method of any one of the previous embodiments, wherein the combination of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream comprises greater than 90 wt % nitrogen, based on the total weight of the combination.

Embodiment 7

The method of any one of the previous embodiments, wherein the combination of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream comprises nitrogen, based on the total weight of the combination, in an amount greater than the nitrogen-containing vent gas stream, based on the total weight of the nitrogen-containing vent gas stream.

Embodiment 8

A system for recovering nitrogen gas from a process gas stream comprising: a process gas stream comprising nitrogen and C₂-C₄ alkene (e.g., ethylene and/or propylene); a first cooling medium stream (e.g., liquid and/or gaseous nitrogen); a first condensate stream comprising impurities; a cooled process gas stream comprising nitrogen and C₂-C₄ alkene (e.g., ethylene and/or propylene); a first gaseous nitrogen stream; a second cooling medium stream (e.g., liquid and/or gaseous nitrogen); a second condensate stream comprising C₂-C₄ alkene (e.g., ethylene and/or propylene); a nitrogen-containing vent gas stream; a second gaseous nitrogen stream; a combined nitrogen stream; a first condenser vessel operated under suitable conditions to produce the first condensate stream comprising impurities, the cooled process gas stream comprising nitrogen and C₂-C₄ alkene (e.g., ethylene and/or propylene), and the first gaseous nitrogen stream, wherein the first condenser vessel comprises: a first heat exchanger; a first inlet for providing the process gas stream; a second inlet for providing the first cooling medium stream; a first outlet for removal of the first gaseous nitrogen stream; a second outlet for removal of the first condensate stream; and a third outlet for removal of the cooled process gas stream; a second condenser vessel operated under suitable conditions to produce the second condensate stream comprising C₂-C₄ alkene (e.g., ethylene and/or propylene), the nitrogen-containing vent gas stream, and second gaseous nitrogen stream, wherein the second condenser vessel comprises: a second heat exchanger; a fourth inlet for providing the cooled process gas stream; a fifth inlet for providing the second cooling medium stream; a fourth outlet for removal of the nitrogen-containing vent gas stream; a fifth outlet for removal of the second gaseous nitrogen stream; and a sixth outlet for removal of the second condensate stream; and a nitrogen vessel comprising at least two of: a seventh inlet for providing the first gaseous nitrogen stream; an eighth inlet for providing the nitrogen-containing vent gas stream; a ninth inlet for providing the second gaseous nitrogen stream; a seventh outlet for removal of the combined nitrogen stream.

Embodiment 9

The system of embodiment 8 further comprising one or more petrochemical vent gas streams and means for combining the one or more petrochemical vent gas streams to form the process gas stream.

Embodiment 10

The system of embodiment 8 or 9, wherein the first heat exchanger and the second heat exchanger are each independently a coil heat exchanger, a shell and tube heat exchanger or a plate heat exchanger.

Embodiment 11

The system of any one of the previous embodiments, wherein the process gas stream comprises at least about 1.0 wt % nitrogen and less than about 80 wt % C₂-C₄ alkene.

Embodiment 12

The system of any one of the previous embodiments, wherein the nitrogen-containing vent gas stream comprises at least about 90 wt % nitrogen, based on the total weight of the nitrogen-containing vent gas stream, and/or the nitrogen vessel comprises greater than 90 wt % nitrogen, based on the total weight of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream.

Embodiment 13

The system of any one of the previous embodiments, wherein nitrogen vessel comprises nitrogen, based on the total weight of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream, in an amount greater than the nitrogen-containing vent gas stream, based on the total weight of the nitrogen-containing vent gas stream. 

What is claimed is:
 1. A method for recovering nitrogen from a process gas stream comprising: cooling the process gas stream comprising nitrogen and C₂-C₄ alkene in a first condenser vessel with a first cooling medium under suitable conditions to produce a first condensate comprising impurities, a first cooled process gas stream comprising nitrogen and C₂-C₄ alkene, and a first gaseous nitrogen stream; cooling the cooled process gas stream in a second condenser vessel with a second cooling medium under suitable conditions to produce a second condensate comprising C₂-C₄ alkene, a nitrogen-containing vent gas stream, and second gaseous nitrogen stream; and combining at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream.
 2. The method of claim 1, wherein the process gas stream comprises one or more petrochemical vent gas streams.
 3. The method of claim 1, wherein the process gas stream comprises at least about 1.0 wt % nitrogen and less than about 80 wt % C₂-C₄ alkene.
 4. The method of claim 1, wherein the impurities are hydrogen, water, C₁-C₂₀ hydrocarbons, and/or oxygenates.
 5. The method of claim 1, wherein the nitrogen-containing vent gas stream comprises at least about 90 wt % nitrogen based on the total weight of the nitrogen-containing vent gas stream.
 6. The method of claim 1, wherein the first condenser vessel and/or the second condenser vessel each independently comprises a coil heat exchanger, a shell and tube heat exchanger or a plate heat exchanger.
 7. The method of claim 1, wherein the first cooling medium and the second cooling medium is liquid nitrogen and/or gaseous nitrogen.
 8. The method of claim 1, wherein the first cooling medium and/or the second cooling medium is provided at a temperature at least about −196° C. and a pressure of less than or equal to about 1500 kPa.
 9. The method of claim 1, wherein the combination of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream comprises greater than 90 wt % nitrogen, based on the total weight of the combination.
 10. The method of claim 1, wherein the combination of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream comprises nitrogen, based on the total weight of the combination, in an amount greater than the nitrogen-containing vent gas stream, based on the total weight of the nitrogen-containing vent gas stream.
 11. The method of claim 1, wherein the C₂-C₄ alkene is ethylene and/or propylene.
 12. A system for recovering nitrogen gas from a process gas stream comprising: a process gas stream comprising nitrogen and C₂-C₄ alkene; a first cooling medium stream; a first condensate stream comprising impurities; a cooled process gas stream comprising nitrogen and C₂-C₄ alkene; a first gaseous nitrogen stream; a second cooling medium stream; a second condensate stream comprising C₂-C₄ alkene; a nitrogen-containing vent gas stream; a second gaseous nitrogen stream; a combined nitrogen stream; a first condenser vessel operated under suitable conditions to produce the first condensate stream comprising impurities, the cooled process gas stream comprising nitrogen and C₂-C₄ alkene, and the first gaseous nitrogen stream, wherein the first condenser vessel comprises: a first heat exchanger; a first inlet for providing the process gas stream; a second inlet for providing the first cooling medium stream; a first outlet for removal of the first gaseous nitrogen stream; a second outlet for removal of the first condensate stream; and a third outlet for removal of the cooled process gas stream; a second condenser vessel operated under suitable conditions to produce the second condensate stream comprising C₂-C₄ alkene, the nitrogen-containing vent gas stream, and second gaseous nitrogen stream, wherein the second condenser vessel comprises: a second heat exchanger; a fourth inlet for providing the cooled process gas stream; a fifth inlet for providing the second cooling medium stream; a fourth outlet for removal of the nitrogen-containing vent gas stream; a fifth outlet for removal of the second gaseous nitrogen stream; and a sixth outlet for removal of the second condensate stream, and a nitrogen vessel comprising at least two of: a seventh inlet for providing the first gaseous nitrogen stream; an eighth inlet for providing the nitrogen-containing vent gas stream; a ninth inlet for providing the second gaseous nitrogen stream; and a seventh outlet for removal of the combined nitrogen stream.
 13. The system of claim 12 further comprising one or more petrochemical vent gas streams and means for combining the one or more petrochemical vent gas streams to form the process gas stream.
 14. The system of claim 12, wherein the first cooling medium is liquid nitrogen or gaseous nitrogen and the second cooling medium is liquid nitrogen.
 15. The system of claim 12, wherein the first heat exchanger and the second heat exchanger are each independently a coil heat exchanger, a shell and tube heat exchanger or a plate heat exchanger.
 16. The system of claim 12, wherein the process gas stream comprises at least about 1.0 wt % nitrogen and less than about 80 wt % C₂-C₄ alkene.
 17. The system of claim 12, wherein the nitrogen-containing vent gas stream comprises at least about 90 wt % nitrogen, based on the total weight of the nitrogen-containing vent gas stream, and/or the nitrogen vessel comprises greater than 90 wt % nitrogen, based on the total weight of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream, and the second gaseous nitrogen stream.
 18. The system of claim 12, wherein nitrogen vessel comprises nitrogen, based on the total weight of at least two of the first gaseous nitrogen stream, the nitrogen-containing vent gas stream and the second gaseous nitrogen stream, in an amount greater than the nitrogen-containing vent gas stream, based on the total weight of the nitrogen-containing vent gas stream.
 19. The system of claim 12, wherein the C₂-C₄ alkene is ethylene and/or propylene 